(a) Technical Field
The present invention relates to controlling a SOC (State of Charge) of a battery, and more particularly, to a hybrid vehicle equipped with a controller capable of preventing over discharge of a battery by a torque reserve control to an engine when an engine clutch slips.
(b) Description of the Related Art
In general, hybrid electric vehicles (HEVs) use an internal combustion engine and an electric motor as power sources which are classified as a parallel type, a serial type, or a mild type on the basis of a drive train structure, and also use a dual clutch transmission (DCT) in which an input shaft of an even gear set and an input shaft of an odd gear set are separately and respectively connected to two clutches.
In particular, the HEV operates either in an EV mode (Electric Vehicle Mode) under which only an electric motor is used as a power source, or in an REV mode (Hybrid Electric Vehicle Mode) under which both an engine and an electric motor are used as power sources, and it is equipped with an engine clutch for connecting/disconnecting the engine and the electric motor to switch between the EV mode and the HEV mode.
For example, an NC (Normally Close) type engine clutch carries out the HEY mode connecting the engine and the electric motor when it does not operate, while it carries out the EV mode disconnecting the engine and the electric motor when it operates.
Typically, driving states of the engine clutch are defined as: a “Slip” state under which a speed difference (rpm) between both ends of the clutch is greater than a certain value and friction of the clutch is initiated; a “Lock-up” state under which there is no speed difference between both ends of the clutch, and thus torque applied to an input shaft is transmitted to an output shaft by 100%; and an “Open” state under which the input and output shafts do not interfere with each other due to their physical separation. In this regard, each of the Slip, Lock-up, and Open states corresponds to one of states of the engine clutch, which are classified using hydraulic pressure of oil in a clutch and rebound pressure of a return spring, a speed difference between both ends of the clutch, input and output torques of the clutch, torque capacity of the clutch, and the like, and those states are determined in a hybrid control unit (HCU) that is an upper level controller of the HEV.
Accordingly, the Slip, Lock-up, and Open states of the engine clutch have influence on switching of EV mode/HEV mode of the HEV.
However, the Slip state of the engine clutch occurs prior to the Lock-up state, and thus puts a battery into a battery charge disable state, and at the same time, it cannot reduce an amount of battery usage even in a SOC that is not more than a certain amount and thus drop of the engine rpm occurs. Further, battery exhaustion that brings the battery SOC to be less than the certain amount or brings the battery to be discharged can disable driving of a hybrid vehicle, especially if the hybrid vehicle is unable to run on engine power alone. This condition may worsen at the time of running on an uphill road on which the vehicle repeatedly moves and stops and in turn slippage of the engine clutch occurs frequently.
In particular, when the vehicle is running on an uphill road, discharge of the battery to a SOC that is less than a certain amount can reduce drivability of the hybrid vehicle, e.g., when running on bendy roads in mountainous regions.